1. Field of the Invention
The present invention relates to a method of producing a grain-oriented magnetic steel sheet suitable for use as the material of a core of an electric machine such as a transformer, electric power generator or the like and, more particularly, to a method of producing a grain-oriented magnetic steel sheet which exhibits a high level of magnetic flux density, as well as very low level of core loss.
2. Description of Related Art
Si-containing grain-oriented magnetic steel sheets having (110) 001! crystal orientation or (100)001! crystal orientation exhibit excellent soft magnetic properties and, hence, are widely used as cores of electric machines which operate under commercial electric power frequency. Grain-oriented magnetic steel sheets for such use are required to produce small core loss, which is generally expressed as W.sub.17/50, which indicates the core loss produced when the steel sheet is magnetized to 1.7 T at a frequency of 50 Hz. The core loss produced by the core of a generator, transformer or the like can be remarkably reduced by using, as the material of the core, a grain-oriented magnetic steel sheet having a low value of W.sub.17/50. Thus, there has been an increasing demand for the development materials having a smaller value of core loss W.sub.17/50.
In general, methods are known for reducing the core loss of the core material, such enhancing electrical resistance by increasing the content of Si for reducing eddy currents, using thinner steel sheets, or by reducing crystal grain size, or by increasing magnetic flux density by enhancing the integrity of crystal grain orientation. The first-mentioned three methods were examined by the present inventors. The method which relies upon increased Si content has a practical limit in that an excessively large Si content impairs rolling characteristics and workability of the material. The method which uses thinner steel sheets also has a practical limit because it tremendously increases the costs of production.
Many studies and proposals have been made in regard to the method for reducing core loss through enhancement of magnetic flux density. For instance, Japanese Patent Publication No. 46-23820, entitled METHOD OF HEAT-TREATING HIGH MAGNETIC FLUX DENSITY MAGNETIC STEEL SHEETS, discloses a method in which Al-containing steel material is hot-rolled and then annealed at a temperature of from 1000.degree. to 1200.degree. C. and at a high temperature, followed by a quenching, so as to cause precipitation of fine AlN. Then, a final cold rolling is conducted at a large rolling reduction of 80 to 95%. It is said that the product steel sheet exhibits an extremely high magnetic flux density of 1.95 T at B.sub.10. AlN which has been finely precipitated and dispersed serves strongly as an inhibitor of growth of primary recrystallization grains. By using this effect, the method permits secondary recrystallization to occur only on crystal nuclei having good orientation, whereby products having well oriented crystalline structure are obtained.
This method, however, tends to allow coarsening of the crystal grains, making it difficult to reduce core loss. In addition, it is not easy stably to obtain high magnetic flux density of the product, because of difficulty encountered in dissolving AlN in the course of annealing after hot rolling.
More specifically, this method essentially requires that finish cold rolling is conducted at a large rolling reduction of 80 to 95%, in order that the growth occurs only on a small number of nuclei which have good orientation, for the purpose of attaining high magnetic flux density. Therefore, the density of generation of secondary crystallization grains is reduced at the cost of achieving high magnetic flux density, with the result that the magnetic properties are rendered unstable due to coarsening of the crystal grains.
Various techniques have been proposed in regard to production of materials using AlN as an inhibitor. For instance, techniques which rely upon aging effected between successive cold-rolling passes are disclosed in Japanese Patent Publication No. 54-23647 entitled METHOD OF HIGH-GRADE UNI-DIRECTIONALLY ORIENTED MAGNETIC STEEL SHEET and Japanese Patent Publication No. 54-13846 entitled COLD ROLLING METHOD FOR PRODUCING HIGH MAGNETIC DENSITY UNI-DIRECTIONALLY ORIENTED SILICON STEEL SHEET HAVING EXCELLENT PROPERTIES. Attempts have been also made for stabilizing magnetic properties of the materials by using a warm-rolling technique, such as that disclosed in Japanese Patent Laid-Open No. 7-32006 entitled METHOD OF COLD-ROLLING GRAIN-ORIENTED SILICON STEEL SHEET AND ROLL COOLING DEVICE FOR COLD ROLLING MILL. These known methods, however, are still unsatisfactory in that they cannot stably provide products having high levels of magnetic flux density. Thus, the above-described problem regarding stability of products of excellent properties still remains unsolved.
Meanwhile, Japanese Patent Publication No. 58-43445, entitled METHOD OF PRODUCING CUBE-EDGE-ORIENTED SILICON STEEL, discloses a method in which specific decarburization annealing is effected on steel containing 0.0006 to 0.0080% of B and not more than 0.0100% of N, so as to achieve a high magnetic flux density of 1.89 T at B.sub.8. This method, however, can offer only an insignificant increase in the magnetic flux density, thus failing to provide any remarkable reduction of core loss and, therefore, has not been put to industrial use. Nevertheless, this method is considered to be advantageous from an industrial point of view, because its method indicates a comparatively high level of stability of magnetic properties of the products.